255.255.255.0 and Subnetting
Subnetsact as virtual fences, splitting a block of IP addresses into smaller units.
This practice relieves online grid congestion and allows for granular access across subnets.
A subnet mask identifies individual subnets.

Lifewire / Joshua Seong
CIDR supports classless IP addressing and associates connection masks with IP connection numbers independent of their traditional class.
internet Classes
TheInterNIC organizationadministers internet domain names and divides IP addresses into classes.